The present invention relates to magneto-optical heads for use in data recording and retrieval systems.
Thermal magnetic recording provides an erasing and rewriting capability. The principle of thermal magnetic recording is based on a characteristic of certain ferromagnetic materials. When the temperature of the material is raised above the Currie temperature, the magnetization of the material can be affected by a small magnetic field. This principle is used for thermal magneto-optics data storage where a laser beam is focused on the recording medium to raise the temperature of the medium above the Currie temperature. A small electro-magnet is placed on the other side of the medium to create a magnetic field near the medium to change the magnetization of the medium. To retrieve information from the medium a laser beam is again focused on the medium but at lower power. Depending on the magnetization of the medium, the polarization of the beam reflected off the medium is either unchanged or rotated by about 0.4 degree. An analyzer (polarizer) inserted before a photodetector allows the detector to sense these two different states of polarization of the returned beam. One method of erasing the recorded information is to first reverse the direction of magnetization of the electro-magnet and then apply a focused laser beam to raise the temperature of the medium to above the Currie temperature to uniformlY magnetize the medium in one direction.
To use the above principle in optical data storage systems an optical head is needed to produce a focused laser beam on the thermal magnetic medium. Moreover, a polarizer is needed to permit the detector to read the information recorded on the medium.
Polarizers needed for the thermal magnetic optical heads are available commercially in two forms. One is a sheet type polarizer based on dichroism, which is the selective absorption of one plane of polarization in preference to the other orthogonal polarization during transmission through a material. Sheet polarizers are manufactured from organic materials which have been imbedded into a plastic sheet. The sheet is stretched, thereby aligning the molecules and causing them to be birefringent, and then dyed with a pigment. The dye molecules selectively attach themselves to the aligned polymer molecules, with the result that absorption is very high in one plane and relatively weak in the other. The transmitted light is then linearly polarized. The optical quality of the sheet type polarizers is rather low. They are used mostly for low power and visual applications.
Another type of polarizer is based on the use of wire grid structures to separate the two orthogonal polarizations. When light radiation is incident on an array of parallel reflective stripes whose spacing is on the order of or less than the wavelength of the radiation, the radiation whose electric vector is perpendicular to the direction of the array is reflected. The result is that the transmitted radiation is largely linearly polarized. The disadvantage of both types of polarizers is that their light efficiencies are typically less than 30%.
A typical optical head for magneto-optic detection is shown in FIG. 1A. A laser is distinguished from ordinary light in that it has a preferred orientation. P-polarized means the orientation is parallel to the paper in FIG. 1A, s-polarized means that the orientation is perpendicular to the paper. Thus, the s and p components are orthogonal. In FIG. 1A a laser source 10 projects a collimated beam through a beam-splitter 12 to an objective lens 14 which focuses the beam on a magneto-optic medium 16. The properties of beam-splitter 12 are that it will pass 80% of the p-polarized light and reflect 100% of the s-polarized light. With the laser diode p-polarized with respect to the beam-splitter, 80% of the light is transmitted to the medium through the objective lens. The remaining 20% is reflected by the beam-splitter and is lost.
When the laser beam is focused on the surface of the medium, the heat generated raises the material to a temperature at which the orientation of the magnetic domain in the recording medium can be reversed by a small external magnetic field. Consequently, information can be recorded by pulsing the laser diode in accordance with the data sequence.
Recorded information can be retrieved by focusing again a low power laser beam at the media. The interaction of the localized magnetic domain orientation with the light beam causes the polarization of the reflected beam to be rotated by a small angle. In other words, the p-polarized light beam, after interacting with the medium, now contains a small s-polarized component. The light vector's direction of rotation depends on the orientation of the magnetic domain on the medium. Thus, the s-component value is either positive or negative, corresponding to +1 or -1. For digital numbers, the -1 corresponds to a digital 0.
When the reflected beam re-enters beam-splitter 12, 100% of the s-polarized component is reflected and only 20% of the p-polarized component is reflected. The 80% of the p-polarized component in the reflected beam is transmitted through beam-splitter 12 and toward the laser diode and is not used. Since both the p and s components are needed by the detectors, part of either the p or s component must be lost since both cannot be split off by a beam-splitter. The p component is chosen to be reduced to increase the value of the small s component relative to the p component. The polarization orientation of the beam reflected by beam-splitter 12 is further rotated by 45 degrees by waveplate 18 before passing through beam-splitter 20 to further increase the value of the s-component relative to the p-component. Beam-splitter 20 has the properties that 100% of the s-polarized component is reflected and 100% of the p-polarized component is transmitted. The direction of rotation of the returned beam can be detected by taking the difference between the light detected by detector 22 and detector 26. Of course, detector 26 has four detectors so that it can perform tracking error and focus error detection in the normal manner for optical heads.
A second possible embodiment is shown in FIG. 2A. A laser source 10, objective lens 14 and magneto-optic media 16 are provided as in FIG. 1A. Beamsplitter 28 has the same properties as beam-splitter 12 in FIG. 1A. As the returned light is reflected by beam-splitter 28, the light is focused by lens 30 through a grating 32 to detectors 34, 36 and 38. Diffraction grating 32, which is preferably produced by holographic methods, is used in place of a beam-splitter and provides three diffracted beams to detectors 34, 36, and 38. Polarizers 40 and 42 in front of detectors 34 and 38, respectively, enable the detection of the direction of rotation of the polarization of the returned beam. As can be seen from FIG. 2B, detectors 34 and 38 are single detectors while detector 36 is a four-quadrant detector for focus and tracking error detection.